At users' demand for an increasingly high rate of wireless access, there emerges the WLAN, which is able to provide high-rate wireless data access in a relatively small area. Various techniques have been used in WLAN, among which a technical standard with more applications is IEEE 802.11b. This standard utilizes the frequency band of 2.4 GHz with a data transmission rate up to 11 Mbps. Other technical standards utilizing the same frequency band include IEEE 802.11g and the Bluetooth, where the data transmission rate of IEEE 802.11g is up to 54 Mbps. There are other new standards such as IEEE 802.11a and ETSI BRAN Hiperlan2 which use the frequency band of 5GHz with the transmission rate up to 54 Mbps as well.
Although there are various standards for wireless access, most WLANs are used for transferring IP data packets. The specific WLAN access standard adopted by a wireless IP network is usually transparent to the upper-level IP. Such a network is usually configured with Access Points for implementing wireless access of a user terminal and with controlling and connecting devices for implementing IP transmission.
Along with the rising and developing of WLAN, focus of research is shifting to the inter-working of WLAN with various wireless mobile communications networks, such as GSM, CDMA, WCDMA, TD-SCDMA, and CDMA2000. In accordance with the 3GPP standards, a user terminal is able to connect with Internet and Intranet as well as the home network and visited network of a 3GPP system via the WLAN access network. To be specific, when getting accessed locally, a WLAN user terminal will get connected with the 3GPP home network via the WLAN access network, as shown in FIG. 2; when roaming, it will get connected with the 3GPP visited network via the WLAN access network. Some entities of the 3GPP visited network are connected with corresponding entities of the 3GPP home network, for instance, the 3GPP Authentication, Authorization and Accounting (AAA) Proxy in the visited network is connected with the 3GPP AAA server in the home network, the WLAN Access Gateway (WAG) in the visited network is connected with the Packet Data Gateway (PDG) in the home network, as shown in FIG. 1. FIG. 1 and FIG. 2 are the schematic diagrams illustrating the networking architectures of a WLAN inter-working with a 3GPP system under roaming and non-roaming circumstances, respectively.
As shown in FIG. 1 and FIG. 2, a 3GPP system primarily comprises Home Subscriber Server (HSS)/Home Location Register (HLR), 3GPP AAA Server, 3GPP AAA Proxy, WAG, PDG, Charging Gateway (CGw)/Charging information Collecting Function (CCF), and Online Charging System (OCS). User terminals, WLAN access network, and all the entities of the 3GPP system together constitute a 3GPP-WLAN inter-working network, which can be regarded as a WLAN service system. In this service system, 3GPP AAA Server is in charge of the authentication, authorization, and accounting of a user, collecting the charging information sent from the WLAN access network and transferring the information to the charging system; PDG is in charge of transmission of user data from the WLAN access network to the 3GPP network or other packet data networks; and the charging system mainly receives and records the user's charging information transferred from the network while OCS instructs the network to transmit online charging information periodically in accordance with the expenses of the online charged users, makes statistics and conducts control.
Under non-roaming circumstances, when a WLAN user terminal desires to get accessed directly to the Internet/Intranet, the user terminal can access to Internet/Intranet via WLAN access network after it accomplishes authentication with AAA server (AS) via WLAN access network. Should the WLAN user terminal desire to get accessed to the service of 3GPP packet switched (PS) domain as well, it may further request the service of Scenario 3 (WLAN 3GPP IP Access) from the 3GPP home network. That is, the WLAN user terminal initiates a service authorization request for Scenario 3 (WLAN 3GPP IP Access) to the AS of the 3GPP home network, which will carry out service authentication and authorization for that request; if the authentication and authorization succeeds, AS will send an access accept message to the terminal and assign a corresponding PDG for the terminal. When a channel is established between the terminal and the assigned PDG, the terminal will be able to get accessed to the service of 3GPP PS domain. Meanwhile, the offline charging system and OCS records the charging information in accordance with the terminal's use of the network.
Under roaming circumstances, when a WLAN user terminal desires to get accessed directly to the Internet/Intranet, it may make a request to the 3GPP home network by way of the 3GPP visited network for access to the Internet/intranet. Should the user terminal also desire to request the service of Scenario 3 (WLAN 3GPP IP Access) to get accessed to the service of the 3GPP PS domain, the terminal needs to initiate via the 3GPP visited network a service authorization process at the 3GPP home network. The authorization is carried out likewise between the terminal and AS of the 3GPP home network. After the authorization succeeds, AS assigns the corresponding home PDG for the terminal, then the user terminal will be able to get accessed to the service of 3GPP PS domain of the home network after it establishes a channel with the assigned PDG via the WAG of the 3GPP visited network.
As shown in FIG. 3, in a 3GPP-WLAN inter-working network, if a WLAN is connected at the same time with a plurality of 3GPP visited networks, that is, a plurality of operating mobile communications networks (3GPP visited networks herein refer to Visited Public Land Mobile Networks (VPLMN)), it will be necessary for a WLAN user terminal to select a desired VPLMN to access after the terminal gets accessed to the WLAN. For instance, in China, a WLAN access network may be connected simultaneously with two operational VPLMN, China Mobile and China Unicom; then a user of China Unicom, after getting accessed via the WLAN, has to instruct the WLAN access network to get it accessed to the operational VPLMN of China Unicom. For another instance, a French user may roam to a WLAN in China, if the home network of the French user has roaming protocols with both China Mobile and China Unicom, then under the circumstances that the WLAN is connected with both China Mobile and China Unicom, the French user will need to select a VPLMN to get accessed after accessing the WLAN. At present, under the above circumstances, a WLAN user terminal informs the WLAN access network of the self-selected network through network-selecting information, where the network-selecting information is the information about the operational mobile communications network that the user terminal currently desires to access, which may be placed in a separately-defined field, or placed in the field of user identification defined in the format of network access identification (NAI).
However, if the current user terminal is under a network environment with a plurality of WLANs existing at the same time, solutions in the prior art can not ensure that the user terminal is able to select the best PLMN, typically the home network of that user terminal. Take as an example the network environment shown as FIG. 4, the WLAN user terminal is in the coverage of WLAN #1, WLAN #2, . . . , WLAN #n at the same time, where WLAN #1 is connected with visited networks of the current user terminal, VPLMN #1 and VPLMN #2, WLAN #2 is connected with the home network and a visited network of the current user terminal, HPLMN and VPLMN #3, . . . In this case, the best selection for the current user terminal is to get accessed to its home network HPLMN via WLAN #2. However, in accordance with the network-selecting solutions in the prior art, this user terminal may probably get accessed via WLAN #1 first while operational networks directly connected with WLAN #1 are all roaming networks of the user terminal, then the terminal will have no chance to select its own home network directly connected with WLAN #2.
Aimed at making a user terminal able to select the best network to access every time, a method of making query of WLAN access networks in turn has been put forward in another patent application: A user terminal that is covered by more than one WLAN may make query of each detected WLAN in turn with the preset initial network-selecting information; if the current WLAN access network is directly connected with the initially selected network, then get accessed to this initially selected network via the current WLAN access network; otherwise make query of the next WLAN in turn, and so on and so forth. Although this method ensures that the PLMN desired by the user, for example, HPLMN, is found as long as it exists, the approach of making query in turn results in slow accessing rate. So far there has been no specific solution put forward to how to make a user terminal, when it desires only to access its HPLMN, able to find, every time at the fastest rate, the WLAN connected with its home network and get accessed via this WLAN.